1. The Field of the Invention
The present invention relates generally to analog circuits. More specifically, the present invention relates to circuits and methods for detecting an envelope in a radio frequency signal.
2. Background and Relevant Art
Access to information is an essential need of mankind. In order to meet this need, information is often conveyed from one location to another. One particularly convenient way to convey information is to electronically transmit a signal representing information. The electronically transmitted signal may then be received by receiver circuitry, which is then capable of interpreting or rendering the information represented by the signal.
Conventional receivers often include a number of analog circuit components, each having a specific purpose. One often-used analog component is called a Radio Frequency (RF) envelope detector. As a radio frequency signal is transmitted, the magnitude of the radio frequency signal RF can vary widely. An RF envelope detector detects the magnitude of the outer bounds of this RF envelope defined by the peak amplitudes in each cycle of oscillation. RF envelope detection is useful, for example, in demodulating an amplitude-modulated signal in which the information is represented by purposefully varying the amplitude of the peaks in the oscillating signal. RF envelope detection is also useful, for example, when detecting signal strength.
Key performance characteristics of an RF envelope detector include current consumption, sensitivity, and dynamic range. Current consumption relates to the amount of current used. Sensitivity is the signal strength of the weakest signal the system can detect with an acceptable signal-to-noise ratio. Dynamic range is the difference between the signal strength of the strongest and weakest signals that the circuit can respond to.
FIG. 3 illustrates a conventional envelope detector 300 in accordance with the prior art. Conventional envelope detection of an RF signal often involves the rectification of the RF signal, followed by low pass filtering the rectified signal. For example, referring to FIG. 3, the signal RF is received at the input terminal 301 and is rectified with rectifying diode 302. The rectified signal is then passed through low pass filter 303 to remove residual RF content. In this case, the low pass filter is simply a resistor 311 and a capacitor 312 coupled in parallel between the output terminal of the rectifying diode 302 and ground. The filtered signal is then provided on the output terminal 304 of the envelope detector 300.
It would be advantageous to reduce current consumption of RF envelope detectors to thereby reduce the power requirements of the RF envelope detector. This would reduce the cost of operating the envelope detector. Furthermore, if the RF envelope detector is operating on battery power, the battery lifetime may be extended.
Furthermore, it would be advantageous to increase sensitivity of the envelope detector so that the envelope detector may operate with very weak signals. It would further be advantageous to increase the dynamic range of the envelope detector.
Accordingly, what would be advantageous is an RF envelope detector that operates at low current, has high sensitivity (also referred to herein as “low signal” sensitivity), and has high dynamic range. It would furthermore be advantageous if the RF envelope detector could be fabricated using conventional CMOS processes, so as to reduce the cost of fabricating the circuitry containing the RF envelope detector.